Do American English speakers who pronounce cot and caught as [kʰɑt] pronounce all, tall, Paul, etc. with the same vowel quality?
If my subjective experience is anything to go by, I feel like I've heard Americans say [ɑ] (similar or identical to the vowel of palm, with no lip rounding) more in cot, caught, don, dawn, stock, stalk, etc., yet [ɔ~ɒ] (with rounding, similar to AmE north but with no R and to RP thought) more in all, tall, Paul, etc.
If this is true, it can be construed that either
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(a) the so-called cot–caught merger is incomplete before tautosyllabic /l/, just like with /ɹ/ (compare thought and north), i.e.
ɔ → ɑ / _ [-liquid]
(If this is true, doll has to be pronounced differently from all because it has /dɒl/ in RP.)
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(b) there is a conditioned allophone of /ɑ/ that occurs before tautosyllabic /l/, i.e.
ɑ → ɔ / _ [+lateral]
Note that dictionaries are most likely of no help, because, in phonemic transcription, whether the vowel is transcribed as /ɑ/ or /ɔ/ is a rather arbitrary choice, unless there is a minimal pair of /ɑl/ and /ɔl/ (which there isn't AFAIK).
Am I right in my assumption, or maybe I'm mishearing the velarization of /l/ as rounding or something?
Best Answer
I don't know of any good summary of this, but I do recall heard accounts from specific speakers who say that they use something like [ɔ] before /l/ as a conditioned allophone of /ɑ/.
I would be very surprised if any speaker had a merger that applied in all environments except before /l/. It seems hard to prove that nobody has this, but I'm not familiar with any speaker who says they have this.
One thing that may be relevant is that even in accents that maintain a phonemic distinction between /ɑ/ and /ɔ/, British English /ɒl/ actually often corresponds to American English /ɔl/. I don't know of any good description of the exact contexts that conditioned this, but my impression is that for many non-cot-caught-merged American English speakers, /ɑl/ has a restricted distribution, occuring mainly or only before vowels. Judging from dictionary transcriptions, /ɔl/ is more-or-less standard (depending somewhat on the particular word) for non-cot-caught-merged American English speakers when "ol" is followed by a consonant or the end of a word (or in other words, in situations where the "l" is unambiguously a coda consonant). See my list of examples in What source explains the different pronunciations of "hol" in "alcohol" and "hollow"?
There is also a word spelled with "aul" where British English speakers all have short /ɒ/, but American English speakers may have either /ɔl/ or /ɑl/: cauliflower. Also, many British English speakers have /ɒl/ in words like vault, fault, false, spelled with "aul" or "al" followed by a voiceless consonant, but as far as I know non-cot-caught-merged American English speakers only have /ɔl/ in these words.
To sum up, I think your option (b) is more likely to be correct. It's true that speakers who don't distinguish caught and cot do maintain a distinction between north and start, but I don't think this is really analogous to a hypothetical merger of caught-cot with maintenance of a distinction between caller and collar (Peter Shor's example of a minimal pair). Rather, I would expect the pre-l context to be one of the first places where complete merger occurs, but it is quite likely that the merged vowel in this position will be realized as [ɔ], since even for non-merged speakers there is a tendency to use the phoneme /ɔ/ rather than /ɑ/ before /l/ in some contexts.